EPR studies of dimerization of Cu(II)porphyrins have been extended to water soluble porphyrin Cu(II) tetra (4-sulfonatophenyl) porphyrin, and further information was obtained for the chemical conditions of dimerization in Cu(II)-cytochrome and its heme peptides. Also, dimer formation of high spin Fe(III)-porphyrins, other than mu-oxo bridged type was established for the first time; the structural characteristics of the dimers and the chemical environment of formation were clarified.